Prandtl layer turbines were first described by Nikola Tesla in U.S. Pat. No. 1,061,206 (Tesla). For this reason, these turbines are sometimes referred to as "Tesla Turbines". FIGS. 1 and 2 show the design for a prandtl layer turbine as disclosed in Tesla. As disclosed by Tesla, a prandtl layer turbine 10 comprises a plurality of discs 12 which are rotatably mounted in a housing 14. Housing 14 comprises ends 16 and ring 18 which extends longitudinally between ends 16. Discs 12 are spaced apart so as to transmit motive force between a fluid in housing 14 and rotating discs 12.
The discs 12, which are flat rigid members of a suitable diameter, are non-rotatably mounted on a shaft 20 by being keyed to shaft 20 and are spaced apart by means of washers 28. The discs have openings 22 adjacent to shaft 20 and spokes 24 which may be substantially straight. Longitudinally extending ring 18 has a diameter which is slightly larger than that of discs 12. Extending between opening 22 and the outer diameter of disc 12 is the motive force transfer region 26.
The transfer of motive force between rotating discs 12 and a fluid is described in Tesla at column 2, lines 30-49. According to this disclosure, fluid, by reason of its properties of adherence and viscosity, upon entering through inlets 30, and coming into contact with rotating discs 12, is taken hold of by the rotating discs and subjected to two forces, one acting tangentially in the direction of rotation and the other acting radially outwardly. The combined effect of these tangential and centrifugal forces is to propel the fluid with continuously increasing velocity in a spiral path until it reaches a suitable peripheral outlet from which it is ejected.
Conversely, Tesla also disclosed introducing pressurized fluid via pipes 34 to inlets 32. The introduction of the pressurized fluid would cause discs 12 to rotate with the fluid travelling in a spiral path, with continuously diminishing velocity, until it reached central opening 22 which is in communication with inlet 30. Motive force is transmitted by the pressurized fluid to discs 12 to cause discs 12 to rotate and, accordingly, shaft 20 to rotate thus providing a source of motive force.
Accordingly, the design described in Tesla may be used as a pump or as a motor. Such devices take advantage of the properties of a fluid when in contact with the rotating surface of the discs. If the discs are driven by the fluid, then as the fluid passes through the housing between the spaced apart discs, the movement of the fluid causes the discs to rotate thereby generating power which may be transmitted external to the housing via a shaft to provide motive force for various applications. Accordingly, such devices function as a motor. Conversely, if the fluid in the housing is essentially static, the rotation of the discs will cause the fluid in the housing to commence rotating in the same direction as the discs and to thus draw the fluid through the housing, thereby causing the apparatus to function as a pump or a fan. In this disclosure, all such devices, whether used as a motor or as a pump or fan, are referred to as "prandtl layer turbines" or "Tesla turbines".
Various designs for prandtl layer turbines have been developed. These include those disclosed in U.S. Pat. No. 4,402,647 (Effenberger), U.S. Pat. No. 4,218,177 (Robel), U.S. Pat. No. 4,655,679 (Giacomel), U.S. Pat. No. 5,470,197 (Cafarelli) and U.S. Reissue Pat. No. 28,742 (Rafferty et al). Most of these disclosed improvements in the design of a Tesla turbine. However, despite these improvements, Tesla turbines have not been commonly used in commercial environment.